1. Field of the Invention
The present invention relates to a retaining substrate for mounting a semiconductor element thereon and a semiconductor device in which said substrate is used. More particularly, the present invention relates to a insulative retaining substrate for mounting a semiconductor element thereon and which has a function of efficiently performing heat radiation for said semiconductor element mounted thereon and a semiconductor device comprising a semiconductor element mounted on said insulative substrate.
2. Related Background Art
In recent years, the global warming of the earth because of the so-called greenhouse effect to an increase in the content of CO2 gas in the air has been predicted.
In view of this, there is an increased demand for the development of clean energy sources with no accompaniment of CO2 gas exhaustion. As one of such clean energy sources, there can be mentioned atomic power generation. However, for the atomic power generation, there are problems which are difficult to be solved, such as radioactive wastes and the like which cause air pollution. Also in view of this, there is an increased demand for providing clean energy sources which are highly safe and do not exhaust air-polluting substances.
Under these circumstances, public attention has now focused on a solar cell which converts sunlight into electric energy as a clean energy source, because it does not exhaust contaminants and it is safe and can be readily handled.
As such solar cell, there are known have a variety of solar cells. And some of them have been using as power generation sources in practice. These solar cells include crystalline series solar cells in which single crystal silicon material or polycrystal silicon material is used, amorphous series solar cells in which amorphous silicon material is used, and compound semiconductor series solar cells in which compound semiconductor material is used. Besides, there are known a variety of configurations for these solar cells to be practically used. Specifically, there are known, for instance, a frame type solar cell as disclosed in Japanese Unexamined Patent Publication No. 82820/1993, a frame-less type solar cell as disclosed in Japanese Unexamined Patent Publication No. 131048/1995, a roofing material-integral type solar cell as disclosed in disclosed in Japanese Unexamined Patent Publication No. 177187/1996 or Japanese Unexamined Patent Publication No. 97727/1999, and an optical-concentration type solar cell as disclosed in Japanese Unexamined Patent Publication No. 83006/1997.
For any of these solar cells, the material cost of the cell (the photoelectric conversion element) constituting the solar cell accounts for the largest rate of the cost of the solar cell. Thus, in order to reduce the cost of the solar cell, it is an important factor to diminish the use amount of the material constituting the cell (the photoelectric conversion element). The optical-concentration type solar cell is of the configuration in that in order to reduce the power generation cost by making full use of the ability of a photoelectric conversion element (a cell) used therein which is costly, sunlight is converged and condensed to several times to several hundreds times by means of a condenser lens to increase the quantity of incident light to the photoelectric conversion element, whereby diminishing the use amount of the photoelectric conversion element.
Specifically, the optical-concentration type solar cell disclosed in Japanese Unexamined Patent Publication No. 83006/1997 is of the configuration in that a solar cell comprising a compound semiconductor material such as GaAs or the like is arranged on a retaining substrate constituted by glass, resin or ceramics, a reverse taper-like concaved portion whose open area being upward widened is arranged above the solar cell, and a light-converging structural body with a high refraction factor and which comprises a resin such as polystyrene and has a surface processed into a lens-like form is accommodated in said concaved portion. Separately, Japanese Unexamined Patent Publication No. 231111/1995 discloses a substrate for an optical-concentration type solar cell. This substrate has a structure in that a plurality of small solar cells are connected respectively to a standard IC-type carrier comprising a dual in-line package or the like and the carriers are attached to a print substrate comprising a throughhole substrate or the like to establish electrical connection between the carriers.
For these solar cells, there is a drawback such that when the temperature of the solar cell is increased, the photoelectric conversion efficiency (the power generation efficiency) is deteriorated. In order to solve such problem, Japanese Unexamined Patent Publication No. 83003/1997 proposes a method in that for a solar cell formed on a substrate, a cooling fin is provided at a said substrate and a ventilation trunk is formed at said substrate to prevent the temperature of the solar cell from being raised.
This cooling method is effective when the solar cell is installed in an environmental atmosphere whose temperature is not remarkably increased. However, in the case of an optical-concentration solar cell, as the light condensation magnitude is increased, the temperature of the solar cell is extremely increased. And it is difficult to sufficiently prevent the extreme temperature rising of the solar cell in this case by aforesaid cooling method.
Detailed description will be made of this point with reference to the drawings.
FIGS. 3(a) to 3(d) are schematic views illustrating the structure of an example of an optical-concentration type solar cell comprising a single crystal silicon material.
Particularly, FIG. 3(a) is a schematic plane view illustrating an appearance of a light receiving face side of the optical-concentration type solar cell, FIG. 3(b) is a schematic view illustrating an appearance of a first side face of the optical-concentration type solar cell, FIG. 3(c) is a schematic view illustrating an appearance of a second side face of the optical-concentration type solar cell, and FIG. 3(d) is a schematic plane view illustrating an appearance of a non-light receiving face side of the optical-concentration type solar cell.
In FIGS. 3(a) to 3(d), reference numeral 1 indicates a photovoltaic element constituted by a single crystal silicon material, and reference numerals 2 and 3 a pair of output electrodes of the photovoltaic element, which are provided on the non-light receiving face side as shown in FIG. 3(d).
In the inside of the photovoltaic element, there is provided a photoelectric conversion semiconductor layer having a plurality of p-n junction structures formed by alternately stacking a p-type semiconductor layer and an n-type semiconductor layer as shown in FIG. 4, where a p-type layer electrode 41 is electrically connected to each p-type semiconductor layer, and an n-type layer electrode 42 is electrically connected to each n-type semiconductor layer. The p-type electrode layers 41 are electrically connected to one of the two output electrodes 2 and 3 of the photovoltaic element and the n-type electrode layers 42 are electrically connected to the other output electrode of the photovoltaic element.
FIGS. 5(a) to 5(d) are schematic views illustrating the structure of an example of an optical-concentration type solar cell comprising a single crystal silicon material and in which light convergence is performed by means of a Fresnel lens.
Particularly, FIG. 5(a) is a schematic plane view illustrating an appearance of a light receiving face side (the light receiving face having a reflection preventive film 51) of the optical-concentration type solar cell, FIG. 5(b) is a schematic view illustrating an appearance of a first side face of the optical-concentration type solar cell, FIG. 5(c) is a schematic view illustrating an appearance of a second side face of the optical-concentration type solar cell, and FIG. 5(d) is a schematic plane view illustrating an appearance of a non-light receiving face side of the optical-concentration type solar cell.
In FIGS. 5(a) to 5(d), reference numeral 1 indicates a photovoltaic element constituted by a single crystal silicon material. On a surface region of the front surface of the photovoltaic element 1 where converged light is irradiated; a reflection preventive film 51 in a round form is provided. Reference numerals 2 and 3 indicate a pair of output electrodes which are provided on the non-light receiving face side of the photovoltaic element as shown in FIG. 5(d).
As well as in the case of the photovoltaic element shown in FIGS. 3(a) to 3(d), in the inside of the photovoltaic element, there are provided a plurality of photoelectric conversion semiconductor layers having a plurality of p-n junction structures formed by alternately stacking a p-type semiconductor layer and an n-type semiconductor layer, where a p-type layer electrode is electrically connected to each p-type semiconductor layer, and an n-type layer electrode is electrically connected to each n-type semiconductor layer. The p-type electrode layers are electrically connected to one of the two output electrodes 2 and 3 and the n-type electrode layers are electrically connected to the other output electrode.
In such optical-concentration type solar cell having such configuration as shown in FIG. 3 [FIGS. 3(a) to 3(d)] or FIG. 5 [FIGS. 5(a) to 5(d)], it is required that converged light is effectively irradiated to the photovoltaic element in the solar cell. In order to satisfy this requirement, it is necessary that the active area of the solar cell is brought unlimitedly near 100%. In view of this, the output electrodes extending from the photovoltaic element in the solar cell are provided on the non-light receiving face side of the solar cell.
The optical-concentration type solar cell having the output electrodes on the non-light receiving face side as above described is mounted on a retaining substrate constituted by glass, resin, ceramics or the like.
Description will be made of an example of such solar cell mounted on a retaining substrate with reference to FIG. 6 [FIGS. 6(a) to 6(c)]. FIG. 6(a) is a schematic view illustrating an appearance of an arrangement state in which a plurality of photovoltaic elements (solar cells) are arranged on such retaining substrate as above described while being serialized with each other, when viewed from above. FIG. 6(b) is a schematic view illustrating a circuit pattern provide on the retaining substrate, when viewed from the right receiving face side of the retaining substrate. FIG. 6(c) is a schematic cross-sectional view, taken along the line Cxe2x80x94Cxe2x80x2 in FIG. 6(a). In FIGS. 6(a) to 6(c), reference numeral 5 indicates a retaining substrate made of ceramics and having a thickness of about 0.5 mm to about 1 mm. Reference numeral 4 indicates a circuit pattern made of copper and having a thickness of about 0.01 mm to about 1 mm, which is formed on the retaining substrate 5. Reference numeral 1 indicates a photovoltaic element (a solar cell) which is arranged on the circuit pattern 4. Upon arranging the photovoltaic element 1 on the circuit pattern 4, there is used a solder. Reference numeral 6 indicates a resist which is formed on the circuit pattern 4 in order to prevent the solder (which is used upon arranging the photovoltaic element 1 on the circuit pattern 4) from being flown to the outside and also in order to decide the arrangement position of the photovoltaic element 1 on the circuit pattern 4. The photovoltaic element 1 is arranged at a prescribed position on the circuit pattern 4 which is decided by the resist 6, and a first output electrode 2 and a second output electrode 3 of the photovoltaic element 1 are electrically connected to the circuit pattern by means of the solder. By the circuit pattern 4 formed on the retaining substrate 5 in this way, a power generated by the photovoltaic element 1 on the retaining substrate 5 is outputted to the outside through the first and second electrodes 2 and 3.
Incidentally, in an optical-concentration type solar cell having such structure as above described, incident sunlight is condensed to several times to several hundreds times by means of a condenser lens. Therefore, it is a general phenomenon that the temperature of a portion of the photovoltaic element in the optical-concentration type solar cell where the sunlight is condensed as above described is extremely increased to reach several hundreds centigrade (xc2x0C.) Thus, the materials used in the photovoltaic element and the structure of the photovoltaic element are required to have excellent heat-resistance and heat radiation characteristics.
In the above-described optical-concentration type solar cell, the positional relation between the retaining substrate 5 having the photovoltaic element mounted thereon and the condenser lens (not shown in the figure) are generally fixed, where in general, there will not be such an occasion that condensed incident sunlight is irradiated to other portion than the photovoltaic element under use environment.
However, for instance, upon performing the maintenance work, there is sometimes occurred necessity to correct the positional relation between the retaining substrate and the condenser lens. At that time, in the case where the positional relation is accidentally made such that condensed incident sunlight is irradiated to the resist 6, because a resin material comprising an epoxy resin or the like is generally used as the resist 6, when aforesaid condensed incident sunlight is irradiated to the resist 6, there will be an occasion in that the resist is markedly deteriorated or it is thermally decomposed to disappear in the worst case. In the case where the resist 6 used in the conventional retaining substrate as shown in FIG. 6 accomplishes its function particularly upon mounting the photovoltaic element on the retaining substrate and has substantially no necessity under use environment. However, there is a problem such that if the resist is thermally decomposed, refuses and the like are provided. In order to prevent the occurrence of this problem, there is considered a method in that the resist 6 is not provided in the retaining substrate 5. However, when this method is adopted, there will be occurred such problems as will be described in the following. For instance, in the case where the photovoltaic element is mounted on the retaining substrate using a reflow solder, the solder is flown out and along with this, the photovoltaic element is moved, where it is difficult to arrange the photovoltaic element at a prescribed position.
The present invention is aimed at solving the foregoing problems in the prior art and providing a retaining substrate for mounting a semiconductor element such as a photovoltaic element thereon, which makes it possible to effectively arrange a photovoltaic element at a prescribed position of the substrate without necessity of using such resist as used in the prior art upon mounting said photovoltaic element, and which has a function to effectively radiate heat of said photovoltaic element, and a semiconductor device comprising a semiconductor element mounted on said substrate.
Another object of the present invention is to provide a retaining substrate for mounting a semiconductor element thereon, having a circuit pattern for said semiconductor element, characterized in that said substrate comprises an insulative material, said substrate has a surface configuration having a cross section comprising (a) an uneven shaped portion and (b) a concave shaped portion which are arranged in tandem wherein a surface of said uneven shaped portion (a) makes a bottom face of said concave portion (b), and said uneven shaped portion (a) has a structure in that (a-i) an insulative convex shaped portion and (a-ii) an insulative concave shaped portion are alternately arranged so as to neighbor with each other, and said circuit pattern is provided in said concave shaped portion (a-ii) of said uneven shaped portion (a).
Specifically, the retaining substrate for mounting a semiconductor element thereon has the uneven shaped portion (a) having a structure in that the insulative convex shaped portion (a-i) and the isulative concave shaped portion (a-ii) are alternately arranged so as to neighbor with each other at a lower portion thereof and has the concave shaped portion (b) above the uneven shaped portion (a). In more detail, the uneven shaped portion (a) has a structure in that a plurality of insulative concave shaped portions (a-ii) each circumscribed by an insulative convex shaped portion (a-i) are arranged to neighbor with each other. A circuit pattern for a semiconductor element is provided in each insulative concave shaped portion (a-ii) and an electrode of a semiconductor element is arranged on the circuit pattern situated in the insulative concave shaped portion (a-ii) such that said electrode is close-contacted with the circuit pattern while being electrically connected with the circuit pattern where the insulative convex shaped portion (a-i) is present between each adjacent insulative concave shaped portions (a-ii). The semiconductor element is arranged in the concave shaped portion (b) situated above the uneven shaped portion (a), where the semiconductor element is close-contacted with the insulative convex shaped portion (a-i) of the uneven shaped portion (a) situated below the concave shaped portion (b) and simultaneously with this, the semiconductor element is close-contacted with the substrate body.
The retaining substrate of the present invention which has such specific structure as above described has significant advantages as will be described below.
It is possible to effectively and desirably mount a semiconductor element having an electrode on the substrate side thereof on the retaining substrate without using a resist (which is inferior in terms of the heat resistance) which is used in the prior art and while preventing occurrence of positional shift of the semiconductor element upon mounting it on the substrate.
It is also possible that heat of the semiconductor element is radiated to the insulative substrate to prevent the temperature of the semiconductor element from being increased. Thus, the characteristics of the semiconductor element are prevented from being deteriorated.
The retaining substrate is effective particularly in the case of a retaining substrate for mounting a semiconductor element thereon, which is heated to a temperature at which a resist is thermally decomposed. Specifically, the present invention is particularly advantageous to be applied in an optical-concentration type solar cell whose temperature is extremely raised due to an external factor by sunlight which is irradiated, where it is substantially impossible to use a resist, because the retaining substrate has excellent heat-radiating function, when the substrate is used as a retaining substrate of the optical-concentration type solar cell, the problems relating to temperature raise thereof can be effectively solved without using any cooling means.
The present invention also makes it a further object to provide a semiconductor device comprising an insulative substrate and a semiconductor element provided on said substrate, said semiconductor element having an electrode, characterized in that said substrate has an uneven shaped portion (a) in that an insulative convex shaped portion (a-i) and an isulative concave shaped portion (a-ii) are alternately arranged so as to neighbor with each other at a lower portion thereof and has a concave shaped portion (b) situated above said uneven shaped portion (a), a circuit pattern for said semiconductor element and said electrode of said semiconductor element are arranged in this order in said insulative concave shaped portion (a-ii) of said uneven shaped portion (a) such that said circuit pattern and said electrode are close-contacted with each other while being electrically connected with each other, and said semiconductor element is arranged in said concave shaped portion (b) situated above said uneven shaped portion (a) while being close-contacted with said insulative convex shaped portion (a-i) of said uneven shaped portion (a) and while being close-contacted with the substrate body.